Got it sorted out; problem was this "Precision Components" 108B speakerphone I used for testing is electrically quite different from a WECO 108A. Apparently it doesn't have the same relay locking circuit the WECO units have, and would turn off when releasing the ON button.

When I swapped out the PC 108B for a regular WECO 108A (just acquired recently from ebay), it all worked fine.

So this is great, seems this custom 82b connection block board is working OK.

I don't have schematics and practice for configuring the PC 108B, but tracing out the circuit a bit, found the relay coil circuit is very unlike the WECO schematics. And from old photo's I've seen from past ebay sales, these came with a connection block similar to the 82b, but must have slightly different wiring to manage locking the relay coil.